Safety devices of the light barrier type are known which enable immediate stoppage of the motion of the machine to be ordered when the presence of a foreign body such, for example, as a tool or the operator's hand is detected in a zone forbidden to access.
Amongst the devices known, an interesting device is described in the French Pat. No. 2,512,920. This device consists of a laser emitter located at one side of the press and emitting a laser beam directed in parallel with the edge of the punch and extending in the immediate vicinity of the vertical plane passing through the bottom edge of the punch, at a distance above the upper surface of the piece to be folded, which lies between the thickness of a finger and the stopping distance of the punch at low speed. If a body is introduced into the danger zone either transversely or pushed in front of the punch as it descends, the travel of the punch is interrupted. The system is automatically put out of action the instant the punch reaches the beam. At this moment the distance between the punch and the upper surface of the metal sheet is such that it is no longer possible to introduce a finger between.
This device has the advantage of allowing the operator great freedom of manipulation of the piece to be folded on the machine, as opposed, for example, to a light curtain insntalled in front of the machine. The efficacy of this device is, however, not complete. In short, in the case of a modern press brake the reaction time of the press is of the order of from 80 to 100 or even 120 milliseconds, older press brakes having stopping times which are sometimes much longer. Now, as the high speed of the slide block reaches nowadays 100 millimeters per second, the resulting stopping distance is of the order of from 8 to 12 millimeters.
The position of the light beam is generally chosen to be 8 millimeters above the upper surface of the piece which is to be folded, so that the operator or anyone else cannot accidentally introduce his fingers under the punch after the safety system has been put out of action. In order that this device shall be effective it is indispensable that the speed of the slide block shall have been commuted from high speed (about 100 millimeters per second) to slow speed (20 millimeters per second as a maximum) before reaching the so-called safety point. At slow speed, as the stopping distance is no more than about 2 millimeters, safety is obtained. On the other hand, if the press has not been adjusted so as to drop to slow speed above the safety point, the folding is then carried out at high speed and the stopping distance is of the order of from 10 to 12 millimeters and safety is then no longer ensured.